This invention relates to a method and device for the injection of samples into the vaporization injector of a gas chromatography analysis apparatus.
Vaporization injectors are well-known in themselves. They are constituted by a vaporization chamber, usually in the form of a heated cylinder, connected at the bottom to the gas chromatography capillary column and closed at the top by a septum designed to be perforated by the sample injection needle. Said needle is fitted to a syringe containing a measured amount of sample, constituted by the substance to be analysed and usually by a solvent for said substance. When the needle is introduced it perforates the septum and penetrates to a predetermined position in the vaporization chamber. The liquid sample is forced by the syringe plunger into said chamber, where it vaporizes before being conveyed into the gas chromatography column by the carrier gas.
Depending on the procedure chosen, only part of the sample (split mode) or all the sample (splitless mode) is introduced into the column. In any event, it is important for the sample to vaporize thoroughly and for the whole within the vaporization chamber. Two different injection techniques have been used to comply with this requirement, both of which present drawbacks.
The first of these techniques, called xe2x80x9ccold-fast injectionxe2x80x9d, requires very rapid introduction of the needle and immediate expulsion of the liquid from the syringe so that when the injector head is maintained in relatively cold conditions, heating of the needle in the vaporization chamber, which is maintained at a high temperature, is limited, and does not cause vaporization of the sample in the needle. This technique ensures that the whole sample measured enters the vaporization chamber, preventing discrimination of high-boiling compounds in the needle but also creating a violent jet of liquid in the chamber which is difficult to vaporize. To aid vaporization, obstacles, usually glass wool, are placed in the chamber; the liquid is deposited on them and then vaporized. However, the material used for these obstacles tends to absorb part of the sample and cause it to degrade, thus distorting the analysis. In addition, the high introduction speeds necessarily require the use of an automatic sampler, as the operation cannot be performed manually in a sufficiently fast and reproducible manner. This means that manual and automatic injections are fundamentally different, and will produce different results.
The second technique currently used involves flash vaporization of the sample in the chamber and preheating of the sample in the needle. This technique requires the needle, emptied of the sample, to be left in the hot vaporization chamber for a period (e.g. 3-5 seconds) before the sample is injected. The period spent by the needle in the chamber increases the temperature in the needle, and this causes a violent evaporation of the part of the solvent in the sample, which passes into the heated part of the needle when the syringe plunger is lowered, thus creating high pressure which expels the liquid from the needle. As the sample is maintained in the syringe while the needle is empty before injection, distillation in the needle, which produces discrimination, can be minimised. On exit from the needle the liquid is sprayed in small droplets which immediately slow to the speed of the carrier, forming a mist. No packing or obstacle is therefore required to stop the jet of liquid. The drawback of this technique is that a very long needle is required, which creates dead volume because the volumetric quantity of the sample injected is not the quantity measured, as in the preceding case, but that quantity plus the volume of the needle.
Another drawback of both techniques is the presence of the septum which, being perforated by the needle, can introduce contaminant particles into the vaporization chamber.
That being said, an object of this invention is to offer a method and device for the vaporization injection of samples in a gas chromatography analysis apparatus which eliminates the drawbacks of the techniques currently used, in particular by optimising the conditions of introduction of the sample into the vaporization chamber, and therefore the conditions of vaporization of the sample, by a procedure involving dispersing the sample in the chamber, which gives rise to rapid vaporization of the sample, thus preventing the sample from remaining in the liquid state as far as possible.
The invention is also designed to prevent discrimination of the sample in the needle and to minimise the dead volume introduced by the needle into the measurement of the sample.
Finally, another object of the invention is to eliminate the drawbacks mentioned above, caused by the presence of the vaporization chamber closing septum, by enabling this septum to be eliminated.
The invention will now be described with reference to embodiments thereof which are illustrated by way of example in the annexed drawings, in which: